Semiconductor device and a method for fabricating the same

ABSTRACT

A semiconductor device includes first-type-channel field effect transistors (FETs) including a first first-type-channel FET including a first gate structure and a second first-type-channel FET including a second gate structure. The first first-type-channel FET has a smaller threshold voltage than the second first-type-channel FET. The first gate structure includes a first work function adjustment material (WFM) layer and the second gate structure includes a second WFM layer. At least one of thickness and material of the first and second WFM layers is different from each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. application Ser.No. 15/355,717 filed on Nov. 18, 2016, which claims priority to U.S.Provisional Patent Application 62/293,636 filed Feb. 10, 2016, theentire disclosure of each of which is incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to a method for manufacturing a semiconductordevice, and more particularly to a structure and a manufacturing methodfor a self-aligned contact structure over source/drain regions.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as a finfield effect transistor (Fin FET) and the use of a metal gate structurewith a high-k (dielectric constant) material. The metal gate structureis often manufactured by using gate replacement technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIGS. 1-6 show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a semiconductor deviceaccording to one embodiment of the present disclosure.

FIG. 7 shows an enlarged exemplary cross sectional view corresponding toarea 61 of FIG. 6.

FIGS. 8A-8C and 9A-9C show enlarged exemplary cross sectional viewscorresponding to area 61 of FIG. 6 for various transistors according tovarious embodiments of the present disclosure.

FIGS. 10A-10D show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a metal gate structureaccording to one embodiment of the present disclosure.

FIGS. 11A-11H show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a metal gate structureaccording to another embodiment of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific embodiments or examples of components andarrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, dimensions of elements are not limited to thedisclosed range or values, but may depend upon process conditions and/ordesired properties of the device. Moreover, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed interposing the first and second features, suchthat the first and second features may not be in direct contact. Variousfeatures may be arbitrarily drawn in different scales for simplicity andclarity.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“made of” may mean either “comprising” or “consisting of.”

FIGS. 1-6 show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a semiconductor deviceaccording to one embodiment of the present disclosure.

First, one or more fin structures 20 are fabricated over a substrate 10.The fin structures include a bottom region and an upper region as achannel region. The substrate is, for example, a p-type siliconsubstrate with an impurity concentration in a range from about 1×10¹⁵cm⁻³ to about 1×10¹⁸ cm⁻³. In other embodiments, the substrate is ann-type silicon substrate with an impurity concentration in a range fromabout 1×10¹⁸ cm⁻³ to about 1×10¹⁵ cm⁻³. Alternatively, the substrate maycomprise another elementary semiconductor, such as germanium; a compoundsemiconductor including Group IV-IV compound semiconductors such as SiCand SiGe, Group III-V compound semiconductors such as GaAs, GaP, GaN,InP, InAs, InSb, GaAsP, AlGaN, AlInAs, AlGaAs, GaInAs, GaInP, and/orGaInAsP; or combinations thereof. In one embodiment, the substrate is asilicon layer of an SOI (silicon-on-insulator) substrate.

To fabricate the fin structures 20, a mask layer is formed over thesubstrate 10 and a patterning operation, including a lithography processand an etching process, is performed on the mask layer, thereby forminga mask pattern 15. The mask pattern 15 includes one or more layers ofinsulating material, such as SiO₂ and SiN. In some embodiments, the maskpattern 15 includes a lower layer made of SiO₂ and an upper layer madeof SiN. The mask layer is formed by thermal oxidation, LPCVD (lowpressure chemical vapor deposition), plasma-CVD, or other suitable filmformation methods.

By using the mask pattern 15 as an etching mask, the substrate 10 istrench-etched to form the fin structures 20, as shown in FIG. 1

After forming the fin structures 20, an isolation insulating layer 30 isformed over the fin structures 20. The isolation insulating layer 30includes one or more layers of insulating materials such as siliconoxide, silicon oxynitride or silicon nitride, formed by LPCVD (lowpressure chemical vapor deposition), plasma-CVD or flowable CVD. Theisolation insulating layer may be formed by one or more layers ofspin-on-glass (SOG), SiO, SiON, SiOCN and/or fluorine-doped silicateglass (FSG).

After forming the isolation insulating layer 30 over the fin structures,a planarization operation is performed so as to remove the upper part ofthe isolation insulating layer 30. The planarization operation mayinclude a chemical mechanical polishing (CMP) and/or an etch-backprocess. Then, the isolation insulating layer 30 is further removed(recessed) so that the upper regions of the fin structure 20 areexposed, as shown in FIG. 2. The exposed portion of the fin structure 20will become a channel region of a FinFET.

As shown in FIG. 3, a dummy gate structure 40 is formed over the exposedfin structures 20. The dummy gate structure 40 includes a dummy gateelectrode layer made of poly silicon and a dummy gate dielectric layermade of silicon oxide. Sidewall spacers (not shown) including one ormore layers of insulating materials are also formed on sidewalls of thedummy gate structure.

To fabricate the dummy gate structure, a blanket layer of the dummy gatedielectric layer is formed over the structure of FIG. 2, and a blanketlayer of the polysilicon layer is formed over the dummy gate dielectriclayer. A mask layer is formed over the polysilicon layer, and apatterning operation is performed on the mask layer, thereby obtaining amask pattern 45. By using the mask pattern 45 as an etching mask, thepolysilicon layer is patterned to form the dummy gate structure 40, asshown in FIG. 3.

In the present disclosure, two fin structures for one dummy gatestructure are shown in FIGS. 1-6. However, the numbers of the finstructures and the dummy gate structure are not limited to that shown inFIGS. 1-6. The dummy gate structure 40 extends in the X direction, whilethe fin structures extend in the Y direction (not shown) and arearranged in the X direction, where the Y direction is perpendicular tothe X direction and the Z direction.

After the dummy gate structure 40 is formed, the fin structure 20 notcovered by the dummy gate structure is recessed below the upper surfaceof the isolation insulating layer 30.

Then, source/drain regions (not shown) are formed over the recessed finstructures by using an epitaxial growth method. The source/drain regionsmay include a strain material to apply stress to the channel region.

Then, an interlayer dielectric layer (ILD) 50 is formed over the dummygate structure 40 and the source/drain region, as shown in FIG. 4. Aftera planarization operation, the dummy gate structure 40 is removed so asto make a gate space 55, as shown in FIG. 5. Then, in the gate space 55,a metal gate structure 60 is formed, as shown in FIG. 6.

FIG. 7 shows an enlarged exemplary cross sectional view corresponding toarea 61 of FIG. 6.

A gate dielectric layer 44 is formed over the upper portion (channelregion) of fin structure 20 after an interfacial layer 42 is formed overthe fin structure 20. The gate dielectric layer 44 includes one or morelayers of metal oxides such as a high-k metal oxide. Examples of metaloxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr,Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb,Lu, and/or mixtures thereof. The interfacial layer 42 is made of, forexample silicon dioxide. The gate dielectric layer 44 is formed by CVD,atomic layer deposition (ALD), or other suitable film formation methods.

Further, a first conductive layer 70, which functions as a barrier orprotective layer in the subsequent etching processes, is formed over thegate dielectric layer 44. The first conductive layer 70 includes one ormore layers of Ti, Ta, TiN and TaN. In one embodiment, a TaN layer isused. In other embodiments, a TaN layer is formed on a Ti layer.

The metal gate structure 60 further includes one or more layers of workfunction adjustment material (WFM) 80 formed over the first conductivelayer 70. A second conductive layer 90 is formed over the WFM layer 80,and then a third conductive layer 95 of a major gate metal layer isformed over the second conductive layer.

The second conductive layer 90 functions as an adhesive layer for thethird conductive layer, and includes one or more layers of Ti, Ta, TiNand TaN. In one embodiment, a TiN layer is used. The third conductivelayer 95 includes a metal material, such as W, Co, Ni, Cu, Al, or alloythereof, or other suitable materials.

The WFM layers 80 includes one or more layers of conductive materialsuch as TiN, TaN, TaAlC, TiC, TaC, Co, Al, TiAl, HfTi, TiSi, TaSi orTiAlC. For the n-channel FET, one or more of TaN, TaAlC, TiN, TiC, Co,TiAl, HfTi, TiSi and TaSi are used as the WFM layers, and for thep-channel FET, one or more of TiAC, Al, TiAl, TaN, TaAlC, TiN, TiC andCo are used as the WFM layers.

Some semiconductor devices include various transistors having differentthreshold voltages. For example, two or three different thresholdvoltages are set for n-channel FETs and two or three different thresholdvoltages are set for p-channel FETs. In the present disclosure, byadjusting the thickness of the WFM layer 80 and/or materials for the WFMlayer 80, threshold voltages (Vt) for various transistors can beadjusted.

FIGS. 8A-8C show exemplary structures of the WFM layer corresponding toarea 61 of FIG. 6 for n-channel transistors according to variousembodiments of the present disclosure.

FIG. 8A is for a first n-channel FET N1 having a threshold voltage Vn1(e.g., ultra-low voltage), FIG. 8B is for a second n-channel FET N2having a threshold voltage Vn2 (e.g., low-voltage), and FIG. 8C is for athird n-channel FET N3 having a threshold voltage Vn3 (e.g., standardvoltage). Here, Vn1<Vn2<Vn3. The three n-channel FETs are disposed in acore area in some embodiments.

In FIG. 8A, the first n-channel FET N1 includes a first WFM layer 100 asthe WFM layer 80. In one embodiment, the first WFM layer 100 includesTiAlC. The thickness of the first WFM layer 100 is in a range from about0.5 nm to 10 nm, in some embodiments, and in a range from about 3 nm toabout 5 nm in other embodiments.

In FIG. 8B, the second n-channel FET N2 includes a second WFM layer 110formed over the first conductive layer 70 and the first WFM layer 100formed over the second WFM layer 110, as the WFM layer 80. In oneembodiment, the second WFM layer 110 includes TiN. It is noted that theWFM layer closest to the channel region of the fin structuresubstantially determines the threshold voltage. The thickness of thesecond WFM layer 110 is in a range from about 0.5 nm to 1.2 nm, in someembodiments.

In FIG. 8C, the third n-channel FET N3 includes a third WFM layer 120formed over the first conductive layer 70, the second WFM layer 110formed over the third WFM layer 110 and the first WFM layer 100 formedover the second WFM layer 110, as the WFM layer 80. In one embodiment,the third WFM layer 120 includes TiN. If any of two adjacent WFM layerare made of the same material, the two adjacent WFM layers can betreated as one “thick” WFM layer. For example, when the third WFM layer120 and the second WFM layer 110 are formed of the same material, thethird WFM layer 120 and the second WFM layer 110 can be treated as one“thick” WFM layer. The thickness of the third WFM layer 120 is in arange from about 0.5 nm to 1.3 nm, in some embodiments.

As shown in FIGS. 8A-8C, the total thickness of the WFM layer 80 for thefirst n-channel FET is the smallest, while the total thickness of theWFM layer 80 for the third n-channel FET is the largest, among the firstto third n-channel FETs.

As set forth above, by adjusting the thickness of the WFM layer 80 andmaterials for the WFM layer 80, threshold voltages (Vt) for the first tothird n-channel FETs can be adjusted to 0<Vn1<Vn2<Vn3.

FIGS. 9A-9C show exemplary structures of the WFM layer corresponding toarea 61 of FIG. 6 for p-channel transistors according to variousembodiments of the present disclosure.

FIG. 9A is a first p-channel FET P1 having a threshold voltage Vp1(e.g., standard voltage), FIG. 9B is a second p-channel FET P2 having athreshold voltage Vp2 (e.g., low voltage), and FIG. 9C is a thirdp-channel FET P3 having a threshold voltage Vp3 (e.g., ultra-lowvoltage). Here, Vp1<Vp2<Vp3<0 V. The three p-channel FETs are disposedin a core area in some embodiments.

In FIG. 9A, the first p-channel FET P1 includes a fourth WFM layer 130formed over the first conductive layer 70, the third WFM layer 120, thesecond WFM layer 110 and the first WFM layer 100, as the WFM layer 80.In one embodiment, the fourth WFM layer 100 includes TiN. The thicknessof the fourth WFM layer 130 is in a range from about 1.0 nm to 2.0 nm,in some embodiments.

In FIG. 9B, the second p-channel FET P2 includes a fifth WFM layer 140formed over the first conductive layer 70, the fourth WFM layer 130, thethird WFM layer 120, the second WFM layer 110 and the first WFM layer100, as the WFM layer 80. In one embodiment, the fifth WFM layer 140includes TiN. The thickness of the fifth WFM layer 140 is in a rangefrom about 0.5 nm to 1.5 nm, in some embodiments.

In FIG. 9C, the third p-channel FET P3 includes a sixth WFM layer 150formed over the first conductive layer 70, the fifth WFM layer 140, thefourth WFM layer 130, the third WFM layer 120, the second WFM layer 110and the first WFM layer 100, as the WFM layer 80. In one embodiment, thesixth WFM layer 150 includes TiN. The thickness of the sixth WFM layer150 is in a range from about 1.0 nm to 2.5 nm, in some embodiments.

If any of two adjacent WFM layer are made of the same material, the twoadjacent WFM layers can be treated as one “thick” WFM layer. Forexample, when the sixth (fifth, fourth or third) to second WFM layersare formed of the same material, the sixth (fifth, fourth or third) tosecond WFM layers can be treated as one “thick” WFM layer.

As shown in FIGS. 9A-9C, the total thickness of the WFM layer 80 for thefirst p-channel FET is the smallest, while the total thickness of theWFM layer 80 for the third p-channel FET is the largest, among the firstto third p-channel FETs. Further, the total thickness of the WFM layer80 for the first p-channel FET is larger than the total thickness of thethird n-channel FET.

As set forth above, by adjusting the thickness of the WFM layer 80 andmaterials for the WFM layer 80, threshold voltages (Vt) for the first tothird p-channel FETs can be adjusted to Vp1>Vp2>Vp3.

In FIGS. 8A-9C, the first n-channel FET includes one WFM layer of afirst material (e.g., TiAlC), and the second and third n-channel FETsand the first to third p-channel FETs include two WFM layers of thefirst material and a second material (e.g., TiN) with differentthicknesses of the second material. In some embodiments, the WFM layerof the first material (100) is not included in at least one of thesecond and third n-channel FETs and the first to third p-channel FETs.In other embodiments, a WFM layer of the second material is formed overthe WFM layer of the first material in the first n-channel FET. Further,in some embodiments, three or more materials are used for the WFMlayers. Yet in other embodiments, the WFM layers 80 for the first tothird n-channel FETs and the first to third p-channel FETs are made ofthe same material with different thicknesses. In such a case, thethickness of the WFM layer 80 for the first n-channel FET N1 issmallest, while the thickness of the WFM layer 80 for the thirdp-channel FET P3 is the largest.

In addition, in some embodiments, the semiconductor device includes twotypes of n-channel FETs with different threshold voltages and/or twotypes of p-channel FETs with different threshold voltages. In otherembodiments, the semiconductor device includes four or more types ofn-channel FETs with different threshold voltages and/or four or moretypes of p-channel FETs with different threshold voltages.

In some embodiments, the three n-channel FETs and three p-channel FETsare disposed in an I/O (input/output) area, in which a thickness of thegate dielectric layer is greater than the core area. In certainembodiments, the thickness of the gate dielectric layer in the I/O areais in a range from about 3 nm to about 6 nm, while the thickness of thegate dielectric layer in the core area is in a range from about 0.5 nmto about 2 nm.

FIGS. 10A-10D show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a metal gate structureaccording to one embodiment of the present disclosure. In FIGS. 10A-10D,the fabrication processes for metal gate structures for n-channel FETsare illustrated, but substantially the same fabrication processes can beapplied to p-channel FETs.

In FIG. 10A, one WFM layer (e.g., the third WFM layer 120) is formedover the first conductive layer 70 for the first to third n-channel FETsN1, N2 and N3, and the third WFM layers 120 formed for the first andsecond n-channel FETs N1 and N2 are removed by dry etching. By the dryetching, the first conductive layers 70 for the first and secondn-channel FETs N1 and N2 are exposed.

In FIG. 10B, another WFM layer (e.g., the second WFM layer 110) isformed over the first conductive layers 70 for the first and secondn-channel FETs N1 and N2 and over the third WFM layer 120 for the thirdn-channel FET N3. Subsequently, the second WFM layer 110 formed over thefirst n-channel FET N1 is removed by dry etching. By the dry etching,the first conductive layer 70 for the first n-channel FET N1 is exposed.

In FIG. 10C, yet another WFM layer (e.g., the first WFM layer 100) isformed over the first conductive layer 70 for the first n-channel FET N1and over the second WFM layers 110 for the second and third n-channelFETs N2 and N3. Subsequently, the third conductive layer 90 and thesecond conductive layer 60 are formed over the first WFM layers 100, asshown in FIG. 10D.

The foregoing fabrication operations include a first operation offorming one or more conductive layers and etching the one or moreconductive layers, thereby exposing the first conductive layer, and asecond operation of forming a conductive layer and not etching theconductive layer. The fabrication operation for forming the WFM layerfor the first n-channel FET N1 includes two first operations and onesecond operation (the WFM layer 100), the fabrication operation forforming the WFM layer for the second n-channel FET N2 includes one firstoperation and two second operations (the WFM layers 100 and 110), andthe fabrication operation for forming the WFM layer for the thirdn-channel FET N3 includes three second operations and no firstoperation.

FIGS. 11A-11H show exemplary cross sectional views illustrating variousstages of the sequential fabrication process of a metal gate structureaccording to one embodiment of the present disclosure. In FIGS. 11A-11H,the fabrication processes for metal gate structures for three n-channelFETs and three p-channel FETs are illustrated.

In FIG. 11A, the sixth WFM layer 150 is formed over the first conductivelayers 70 for the first to third n-channel FETs N1, N2 and N3 and thefirst to third p-channel FETs P1, P2 and P3. Then, the sixth WFM layer150 formed for the second p-channel FET P2 is removed by dry etching. Bythe dry etching, the first conductive layer 70 for the second p-channelFET P2 is exposed.

In FIG. 11B, the fifth WFM layer 140 is formed over the first conductivelayer 70 for the second p-channel FET P2 and over the sixth WFM layers150 for the first to third n-channel FETs N1, N2 and N3 and the firstand third p-channel FETs P1 and P3. Subsequently, the fifth WFM layers140 and the sixth WFM layers 150 formed for the first to third n-channelFETs N1, N2 and N3 and the first p-channel FET P2 are removed by dryetching, as shown in FIG. 11C. By the dry etching, the first conductivelayers 70 for the first to third n-channel FETs N1, N2 and N3 and thefirst p-channel FET P2 are exposed.

In FIG. 11D, the fourth WFM layer 130 is formed over the firstconductive layers 70 for the first to third n-channel FETs N1, N2 and N3and the first p-channel FET P2, over the sixth WFM layer 150 formed forthe second p-channel FET P2 and over the fifth WFM layer 140 formed forthe third p-channel FET P3. Subsequently, the fourth WFM layer 130formed for the third n-channel FET N3 is removed by dry etching, asshown in FIG. 11D. By the dry etching, the first conductive layer 70 forthe third n-channel FET N3 is exposed.

In FIG. 11E, the third WFM layer 120 is formed over the first conductivelayer 70 for the third n-channel FET N3 and over the fourth WFM layers130 formed for the first and second n-channel FETs N1 and N2 and thefirst to third p-channel FETs P1, P2 and P3. Subsequently, the third WFMlayers 120 formed for the first and second n-channel FETs N1 and N2 areremoved by dry etching, as shown in FIG. 11F. By the dry etching, thefirst conductive layers 70 for the first and second n-channel FETs N1and N2 are exposed.

In FIG. 11G, the second WFM layer 110 is formed over the firstconductive layers 70 for the first and second n-channel FETs N1 and N2and over the third WFM layers 120 formed for the third n-channel FET N3and the first to third p-channel FETs P1, P2 and P3. Subsequently, thesecond WFM layer 110 formed for the first n-channel FET N1 is removed bydry etching, as shown in FIG. 11G. By the dry etching, the firstconductive layer 70 for the first n-channel FET N1 is exposed.

In FIG. 11H, the first WFM layer 100 is formed over the first conductivelayer 70 for the first n-channel FET N1 and over the second WFM layers110 for the second and third n-channel FETs N2 and N3 and the first tothird p-channel FETs P1, P2 and P3. Subsequently, the third conductivelayer 90 and the second conductive layer 60 are formed over the firstWFM layers 100, as shown in FIG. 11H.

The foregoing fabrication operations include a first operation offorming one or more conductive layers and etching the one or moreconductive layers, thereby exposing the first conductive layer, and asecond operation of forming a conductive layer and not etching theconductive layer.

The fabrication operation for forming the WFM layer for the firstn-channel FET N1 includes three first operations and one secondoperation (the WFM layer 100), the fabrication operation for forming theWFM layer for the second n-channel FET N2 includes two first operationsand two second operations (the WFM layers 100 and 110), and thefabrication operation for forming the WFM layer for the third n-channelFET N3 includes two first operations and three second operations (theWFM layers 100, 110 and 120).

The fabrication operation for forming the WFM layer for the firstp-channel FET P1 includes one first operation and four second operations(the WFM layers 100, 110, 120 and 130), the fabrication operation forforming the WFM layer for the second p-channel FET P2 includes one firstoperation and five second operations (the WFM layers 100, 110, 120, 130and 140), and the fabrication operation for forming the WFM layer forthe third p-channel FET P3 includes no first operation and six secondoperations (the WFM layers 100, 110, 120, 130, 140 and 150).

In the foregoing fabrication operations, the WFM layers can be formed byCVD, physical vapor deposition (PVD) including sputtering, atomic layerdeposition (ALD), electroplating, or other suitable film formingmethods.

The various embodiments or examples described herein offer severaladvantages over the existing art. For example, in the presentdisclosure, by adjusting the thickness of the WFM layer and/or materialsfor the WFM layer, threshold voltages (Vt) for various transistors canbe adjusted. For example, an offset (a difference) of threshold voltagesbetween a low voltage transistor and an ultra-low voltage transistor canbe reduced by about 7% for n-channel FET and by about 11% for p-channelFET compared with a conventional technology with a single thickness WFMlayer. In some embodiments, the offset for the n-channel FET is 2% ofless and the offset for the p-channel FET is less than about 4%.Further, in the fabrication operations of the present disclosure, thenumber of exposure times of the first conductive layer 70 is at mostthree, and thus it is possible to minimize the thickness of the firstconductive layer 70. Moreover, the dry etching of the WFM layer(s) isperformed on at most two WFM layers, and therefore the etching damage tothe first conductive layer 70 can be minimized.

It will be understood that not all advantages have been necessarilydiscussed herein, no particular advantage is required for allembodiments or examples, and other embodiments or examples may offerdifferent advantages.

According to one aspect of the present disclosure, a semiconductordevice includes first-type-channel field effect transistors (FETs)including a first first-type-channel FET including a first gatestructure and a second first-type-channel FET including a second gatestructure. The first first-type-channel FET has a smaller thresholdvoltage than the second first-type-channel FET. The first gate structureincludes a first work function adjustment material (WFM) layer and thesecond gate structure includes a second WFM layer. At least one ofthickness and material of the first and second WFM layers is differentfrom each other

According to another aspect of the present disclosure, a semiconductordevice includes a first n-channel FET including a first gate structureand having a threshold voltage Vn1, a second n-channel FET including asecond gate structure and having a threshold voltage Vn2, a thirdn-channel FET including a third gate structure and having a thresholdvoltage Vn3, a first p-channel FET including a fourth gate structure andhaving a threshold voltage Vp1, a second p-channel FET including a fifthgate structure and having a threshold voltage Vp2, and a third p-channelFET including a sixth gate structure and having a threshold voltage Vp3.Vn1<Vn2<Vn3 and Vp1<Vp2<Vp3. The first gate structure includes a firstwork function adjustment material (WFM) layer, the second gate structureincludes a second WFM layer, the third gate structure includes a thirdWFM layer, the fourth gate structure includes a fourth WFM layer, thefifth gate structure includes a fifth WFM layer, and the sixth gatestructure includes a sixth WFM layer. At least one of thickness andmaterial of the first to sixth WFM layers is different from each other.

In accordance with yet another aspect of the present disclosure, in amethod of manufacturing gate structures for plural field effecttransistors (FETs) in a semiconductor device, a gate dielectric layer isformed over each of channel layers for the FETs. A first conductivelayer is formed over the gate dielectric layer. A work functionadjustment material (WFM) layer is formed over the first conductivelayer. A second conductive layer is formed over the WFM layer. Theforming the WFM layer for at least one FET includes at least one firstoperation of forming one or more conductive layers and etching the oneor more conductive layers, thereby exposing the first conductive layer,and the forming the WFM layer for at least one FET includes at least onesecond operation of forming a conductive layer and not etching theconductive layer.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A semiconductor device comprising:first-type-channel field effect transistors (FETs) including a firstfirst-type-channel FET including a first gate structure and a secondfirst-type-channel FET including a second gate structure, wherein: thefirst first-type-channel FET has a smaller threshold voltage than thesecond first-type-channel FET, the first gate structure includes a firstwork function adjustment material (WFM) layer and the second gatestructure includes a second WFM layer, and at least one of thickness andmaterial of the first and second WFM layers is different from eachother.
 2. The semiconductor device of claim 1, wherein: thefirst-type-channel FETs further comprises a third first-type-channel FETincluding a third gate structure, the third first-type-channel FET has alarger threshold voltage than the second first-type-channel FET, thethird gate structure includes a third WFM layer, and at least one ofthickness and material of the first to third WFM layers is differentfrom each other.
 3. The semiconductor device of claim 2, wherein: thefirst WFM layer includes a first layer made of a first material, each ofthe second and third WFM layers includes a first layer made of the firstmaterial and a second layer made of a second material different from thefirst material, the first layer being formed over the second layer. 4.The semiconductor device of claim 3, wherein a thickness of the secondlayer of the second WFM layer is smaller than a thickness of the secondlayer of the third WFM layer.
 5. The semiconductor device of claim 4,wherein the second material is TiN.
 6. The semiconductor device of claim2, wherein the first to third WFM layers are formed over a firstconductive layer disposed over a gate dielectric layer.
 7. Thesemiconductor device of claim 6, wherein the first conductive layerincludes a TaN layer.
 8. The semiconductor device of claim 2, wherein:each of the first to third WFM layers includes a layer made a secondmaterial, and a thickness of the layer of the first WFM layer is smallerthan a thickness of the layer of the second WFM layer, and the thicknessof the layer of the second WFM layer is smaller than a thickness of thelayer of the third WFM layer.
 9. The semiconductor device of claim 8,wherein: each of the first to third WFM layers further includes a layermade a first material, and the layer made of the first material isformed over the layer of the second material.
 10. The semiconductordevice of claim 8, wherein the second material is TiN.
 11. Thesemiconductor device of claim 8, wherein the layer of the secondmaterial is formed over a first conductive layer disposed over a gatedielectric layer.
 12. The semiconductor device of claim 3, wherein thefirst material is TiAlC.
 13. A semiconductor device, comprising: a firstn-channel FET including a first gate structure and having a thresholdvoltage Vn1; a second n-channel FET including a second gate structureand having a threshold voltage Vn2; a third n-channel FET including athird gate structure and having a threshold voltage Vn3; a firstp-channel FET including a fourth gate structure and having a thresholdvoltage Vp1; a second p-channel FET including a fifth gate structure andhaving a threshold voltage Vp2; and a third p-channel FET including asixth gate structure and having a threshold voltage Vp3, wherein:Vn1<Vn2<Vn3 and Vp1<Vp2<Vp3, the first gate structure includes a firstwork function adjustment material (WFM) layer, the second gate structureincludes a second WFM layer, the third gate structure includes a thirdWFM layer, the fourth gate structure includes a fourth WFM layer, thefifth gate structure includes a fifth WFM layer, and the sixth gatestructure includes a sixth WFM layer, and at least one of thickness andmaterial of the first to sixth WFM layers is different from each other.14. The semiconductor device of claim 13, wherein: the first WFM layerincludes a first layer made of a first material, and each of the secondto sixth WFM layers includes a first layer made of the first materialand a second layer made of a second material different from the firstmaterial, the first layer being formed over the second layer.
 15. Thesemiconductor device of claim 14, wherein a thickness of the secondlayer increases in the order of the second WFM layer, the third WFMlayer, the fourth WFM layer, the fifth WFM layer and the sixth WFMlayer.
 16. The semiconductor device of claim 15, wherein the secondmaterial is TiN.
 17. The semiconductor device of claim 13, wherein: thefirst to sixth WFM layers are formed over a first conductive layerdisposed over a gate dielectric layer, and a second conductive layer isdisposed over each of the first to sixth WFM layers with a thirdconductive layer interposed therebetween.
 18. The semiconductor deviceof claim 14, wherein the first material is TiAlC.
 19. A semiconductordevice comprising: a first n-channel fin filed effect transistor(FinFET) including a first gate structure; and a second n-channel FinFETincluding a second gate structure, wherein: the first n-channel FinFEThas a smaller threshold voltage than the second n-channel FinFET, thefirst gate structure includes a first work function adjustment material(WFM) layer and the second gate structure includes a second WFM layer,and the first WFM layer includes a TiAlC layer and a TiN layer disposedon the TiAlC layer, and the second WFM layer includes a first TiN layer,a TiAlC layer disposed on the first TiN layer and a second TiN layerdisposed on the TiAlC layer.
 20. The semiconductor device of claim 19,wherein a thickness of the TiN layer of the first WFM layer is equal toa thickness of the second TiN layer of the second WFM.